In electrostatic drum printing on sheets, such as in same facsimile machines, an electrostatic latent image of the sheets is developed into a visible image, usually by applying a liquid suspension of toner particles to the sheets. See for example, U.S. Pat. No. 3,656,173 to R. Fussel. The developer typically must remain on contact with the sheets for about one second to obtain in adequate image contrast. Attempts have been made to increase the rate of printing by increasing the speed of rotation of the drum. However, this sometimes results in a loss of image contrast or density.
In U.S. Pat. No. 4,482,242 to Moraw et al., a latent charge image on a drum is developed by moving the drum through a pool of developer liquid in a developing shoe. The excess liquid is removed by spillage off the edges of the shoe, and by a fast moving roller contacting the drum.
U.S. Pat. No. 4,270,859 to Galbraith et al. provides a pre-wet shoe for depositing a liquid dispersant onto photoconductive paper before development, and a toner shoe adjacent to the paper for applying toner to a latent image on the paper. Toner is applied across the width of an entire sheet. The toner fluid flows in the shoe to the paper, then downhill along the paper to an outlet of the shoe. The toning shoe is raised by a lifting mechanism to a prescribed distance from the drum surface, and the shoe does not contact the drum.
As toner fluid flows in a shoe downhill along the paper, toner particles are deposited on the paper to develop the latent image and the concentration of toner particles in the fluid decreases accordingly. A deposition rate for toner particles that is proportional to the concentration of toner particles in the fluid results in an exponential decrease in toner particle concentration with distance along the shoe. Accordingly, most of the toner particles are applied to the latent image near the beginning of the flow, and depending on the particle deposition rate, the effective length of the toning shoe is reduced to about 1/4 to 1/2 of the actual toning shoe length. In some cases, inadequate image contrast may result. Toner deposition by the toning shoe may be increased by starting with a greater concentration of toner particles, but this can result in background staining or image smearing. Alternatively, the deposition rate may be decreased so that particles are deposited over a greater length of the shoe, but this has no benefit for the image contrast. Increasing the actual length of the toning shoe has little effect on increasing the effective length of the toning shoe, and increasing the drum speed can have a deleterious effect on image contrast and density.
It is also desirable to devise a toning shoe or "development electrode" that allows the use of a print head with helical stripe scanning thereby obtaining a faster printing speed. An even faster print speed could be obtained if the time of fluid developer contact with the sheet could be reduced to less than a full second. However, it is imperative that vital image contrast not be lost. Also streaking and uneven developing must be avoided.
An object of the invention is to devise a developer applicator for a supported latent image, particularly a structure which allows rapid helical stripe pattern printing with improved image contrast and density without streaking or uneven imaging at stripe edges.